1. Field of the Invention
The present invention relates to a sheet feeding apparatus used with an image forming apparatus such as an electrophotographic copying machine. More particularly, it relates to an air sheet feeding apparatus for separating and supplying a sheet one by one from a sheet stack rested on a sheet tray by utilizing an air suction force.
2. Related Background Art
Among conventional sheet feeding apparatuses used an image forming apparatus such as an electrophotographic copying machine and the like, a sheet feeding apparatus of roller type wherein sheets stacked on a sheet tray are separated one by one by rotation of a sheet supply roller and the separated sheet is conveyed downstreamly has widely been used. In such a conventional sheet feeding apparatus, a surface of the sheet supply roller is formed from an elastic body such as rubber having high coefficient of friction which mainly determines feeding ability of the roller. Accordingly, the feeding ability of the roller was unstable since the coefficient of friction of the roller surface was changed because of the change in configuration of the roller due to wear, deterioration of material of which the roller is made and/or adhesion of paper powder to the roller surface, and the conventional roller could not cope with various kinds of sheets having different surface features. To eliminate the above drawbacks, there has been proposed an air sheet feeding apparatus wherein a sheet is absorbed and conveyed by utilizing a suction force of air.
FIG. 11 shows a typical conventional air sheet feeding apparatus. In FIG. 11, a plurality of sheets S0 are stacked on a sheet tray 101 in such a manner that tip ends of the sheets S0 are abutted against a sheet align guide plate 112. A sheet convey portion 100 is arranged above the sheet tray 101 on which the sheets are stacked. The sheet convey portion 100 includes a sheet absorb portion 102, a convey belt 103 having a plurality of absorb holes 103a, a belt drive roller 104 for driving the convey belt 103 in a direction shown by the arrow A, a driven roller 111 on which the convey belt 103 is mounted, and a blower 104 for sucking air through the absorb holes 103a and the sheet absorb portion 102.
The suction of the blower 104 is turned ON/OFF by a valve 109. In order to float the sheets, there is arranged a nozzle 105 for injecting air toward a tip end of the sheet stack rested on the sheet tray obliquely and upwardly in a direction shown by the arrow B. The nozzle 105 is connected to a blower 107 so that the air is supplied to the nozzle.
Such an air sheet feeding apparatus is operated as follows. First of all, the air is injected from the nozzle 105 under the action of the blower 107 to float several sheets from the sheet stack rested on the sheet tray. Then, the uppermost sheet is absorbed to the convey belt 103 under the action of the blower 104 by opening the valve 109. Thereafter, the convey belt 103 is driven to convey the sheet downstreamly to a pait of convey rollers 108. Thereafter, the valve 109 is closed and the convey belt 103 is stopped.
However, in the above-mentioned conventional sheet feeding apparatus, when a first sheet S1 is supplied, there arose the following problem after a trailing end of the first sheet S1 has passed through below the driven roller 111. That is to say, a second sheet S2 is excessively floated by the action of the nozzle 105, so that, as shown in FIG. 12, before the feeding of the first sheet S1 is finished, the second sheet S2 is contacted with the moving convey belt 103 which is not covered by the first sheet S1, with the result that the tip end of the second sheet is strongly struck against the sheet align guide plate 112.
Consequently, the tip end portion of the second sheet S2 is bent or folded. Accordingly, when the second sheet S2 is supplied, the tip end portion of the second sheet cannot be absorbed to the sheet absorb portion 102 completely, thereby causing the poor sheet feeding. This problem frequently occurred in light sheets having relatively small rigidity.